1. Field of the Invention
The present invention relates to a servo control device which controls a machine tool or other machine which uses a motor, in particular a servo control device which reduces the “deflection” of a processing point when using a tool to process a workpiece, that is, a front end part of the tool (a front end point of the machine), as a driven member which the motor drives.
2. Description of the Related Art
In general machine tools etc., as control systems for axes which are driven by motors, there are semi-closed systems which use detectors which are provided at motors for control and full-closed systems which have separate detectors close to the front end parts of the tools (hereinbelow, referred to as the “front end points of the machines”).
In general, due to cost demands, the former semi-closed systems are more frequently used. In such a system, when the rigidities of the motor and the front end point of the machine are low, there is a problem that elastic deformation of the machine causes “deflection” or “torsion” to occur and error arises in the movement of the front end point of the machine with respect to a command.
On the other hand, in machines in which higher precision control is necessary, full-closed systems are used. By arranging a detector near the front end point of a machine, it is possible to reduce the error between the command and movement of the front end point of the machine. However, in general, it is difficult to arrange a detector at the tool front end part or other position close to the front end point of the machine. Even in a full-closed system, there is sometimes error in movement of the front end point of the machine with respect to a command.
As explained above, when the rigidity of the machine is low, not only in a semi-closed system, but also a full-closed system, the “deflection” or “torsion” caused by elastic deformation of the machine sometimes causes error in movement of the front end point of the machine with respect to a command. If such error occurs, the processing precision deteriorates, therefore control is sought which minimizes the error.
In the past, to deal with the “deflection” or “torsion” of the front end point of the machine due to such elastic deformation, two major methods have been proposed. One is the control method using feedback which uses the feedback of a detector of the motor or feedback of an acceleration sensor which can be arranged at a position close to the front end point of the machine so as to estimate movement of the front end point of the machine and uses the results of estimation to control the motor. The other is the control method which uses feedforward to predict the “deflection” or “torsion” from the machine rigidity to correct the commands or the method of restricting acceleration etc. of a command and correcting the command so that no deflection of the front end point of the machine occurs.
For example, the method is known of arranging an acceleration sensor close to the front end point of the machine, estimating the “deflection” or “torsion” of the front end point of the machine from the feedback, and using learning control for control so that the error between the command and movement of the front end point of the machine becomes minimal (for example, refer to Japanese Patent Publication of Un-examined application No. 2006-172149 A). Furthermore, a method has also been disclosed of estimating the “deflection” or “torsion” of the front end point of the machine from the feedback of a detector of a motor without using an acceleration sensor and using learning control for control so that the error between the command and the movement of the front end point of the machine becomes minimum. In this prior art, as the method of reducing the error, learning control is used, therefore in all of the above methods, repetition of commands is required. Therefore, there is a problem that the learning control is not usable with normal commands.